The present invention relates to apparatus and methods for conveying bulk materials in the form of particles through a conduit, such as a pipeline or hose, and more particularly to apparatus and methods in which the particulate material is first transported at high speed through a barrel by an auger conveyer, and then inserted within an envelope of pressurized flowing gas adjacent to the exit end of the barrel to continuously pneumatically propel the material through the conduit.
Pneumatic conveying systems having auger conveyor sections are known in the art and have been in use for many years for transporting various particulate materials, such as pulverized coal, portland cement, grain, pulverized ore, etc., through pipelines and other conduits. Such systems, which are described, for example, in U.S. Pat. Nos. 1,553,539, 1,941,512, 1,941,573, 2,299,470, 3,370,890, 3,602,552 and 3,693,842, typically comprise a motor driven auger conveyor rotatably mounted within a barrel, a gravity-fed hopper for supplying particulate material to the auger conveyor through an opening in the barrel, and a mixing chamber situated at the discharge end of the barrel where the material ejected by the auger conveyor is subjected to one or more jets of pressurized air or other gas. The mixture of material and gas thus formed is propelled by the pressurized gas through a conduit connected to the mixing chamber.
Conventional pneumatic conveying systems with auger conveyor sections have the drawback in that the mixing of the pressurized gas with the particulate material in the mixing chamber produces considerable turbulence in the mixture transported through the conduit. Such turbulence is undesirable in that it increases the frequency of contact of the particulate material with the conduit walls during transport and therefore aggravates frictional losses in the conveying system. Where the material being transported is abrasive or corrosive, increased contact of such material with the conduit walls can also reduce the useful life of the conduit.
Pneumatic conveying systems for particulate materials have also been devised in which the mixing of material with the gas takes place in a venturi pipe through which pressurized gas flows. By introducing the particulate material into the relatively high velocity gas flow in the constriction or throat of the venturi pipe, mixing of the material with the gas occurs with little turbulence, and therefore the resulting mixture may be propelled through the conduit with reduced frictional loss and less wear of the conduit walls.
Conventional venturi conveying systems, such as those described in U.S. Pat. Nos. 3,186,769 and 4,009,912, have also been in use for many years in a variety of conveying applications. In such systems, the particulate material to be mixed with the gas is supplied to the venturi pipe via a chute having a discharge end located near the constriction of the venturi pipe, where a vacuum (negative pressure) created by the high velocity gas flow therein acts to draw the material from the chute into the venturi pipe. Because the material being transported does not always flow freely in the chute, the transport rate in a conventional venturi conveying system is difficult to control and is subject to variations on account of changes in the properties of the material being conveyed, such as density, moisture content, particle size, etc. Moreover, a conventional venturi conveying system may not provide a satisfactory material-to-air conveyance ratio for materials that are not sufficiently free flowing in the chute to allow the negative pressure in the venturi pipe to draw such materials from the chute at an adequate rate.
Both the conventional auger conveyor type and the conventional venturi type pneumatic conveying systems are subject to the problem of "blowback", which occurs when there is a blockage in the conduit that results in excessive pressure in the conduit and produces an undesirable back flow of the gas and product into the hopper in the auger conveyor type system and into the chute in the venturi type system. Known solutions to the blowback problem have included the use of a flapper valve or an air lock in the auger barrel or in the chute of the venturi type system to prevent back flow of gas and product when the pressure in the conduit becomes excessively high. These mechanical devices for avoiding blowback have the disadvantage in that they are subject to binding, clogging and wear from contact with abrasive or corrosive materials, and therefore require substantial maintenance.
Another known solution to the blowback problem used in auger conveyor type systems provides for tapering of the barrel near the discharge end or alterations in the flights of the auger to produce a plug of material which seals the barrel against back flow of the gas. Such modifications to the barrel or the auger flights for forming product-plug seals have the disadvantage, however, in that they tend to reduce the rate at which the material may be transported or, in the case of the modifications to the auger flights, must be tailored to the particle size of the material being conveyed. Maintaining a product plug seal also causes increased energy consumption.
Accordingly, a need exists for a pneumatic conveying system for particulate materials which minimizes turbulence in the material-gas mixture being transported by the conduit and which provides continuous transport of the material at an easily controlled constant rate and satisfactory material-to-air conveyance ratio for a wide range of material properties, including materials that are not freely flowing. Furthermore a need exists for a pneumatic conveying system in which blowback can be prevented without using mechanical expedients such as flapper valves, air locks, or an auger conveyor that produces a product-plug seal.